1. Field of the Invention
The present invention relates to a method for measuring the wavefront aberration of an optical system.
2. Description of the Related Art
An exposure apparatus which transfers the pattern of an original onto a substrate has been conventionally employed in manufacturing, for example, a semiconductor device by photolithography. Since the exposure apparatus may have to precisely transfer the pattern of an original onto a substrate as the exposure target body at a predetermined magnification, it is important to use a projection optical system which has small aberrations and thus is excellent in imaging performance. The importance of such a projection optical system is great especially in recent years, in which the pattern to be transferred is becoming more sensitive to the aberration of the projection optical system in response to a demand for further advances in micropatterning of a device pattern. This makes it necessary to measure the optical performance (e.g., the wavefront aberration) of the projection optical system with high accuracy. The simplification, speed-up, and cost reduction of the measurement are also important from the viewpoint of improving the productivity and economical efficiency of the exposure apparatus.
One conventional method observes by, for example, a scanning electron microscope (SEM) a resist pattern formed by projecting the pattern of an original onto a substrate coated with a resist to expose the resist, and developing the resist. This method takes a long time for processing operations such as exposure and development, so its reproducibility is poor due to errors generated in the process of resist coating and development.
A PDI (Point Diffraction Interferometer) having a pinhole to form an ideal spherical wave has been known. A shearing interferometer which exploits shearing interferometry has been known as well. Nowadays, a measurement apparatus which uses an LDI (Line Diffraction Interferometer) having a slit to form an ideal cylindrical wave or an ideal elliptical wave has been proposed.
In a wavefront aberration measurement apparatus of the LDI scheme, a measurement mark is formed on a plate set behind an optical system to be measured. The measurement mark includes a slit to form an ideal wavefront, and a window which transmits a wavefront which bears the aberration information of the optical system to be measured. A two-dimensional light-receiving element is set directly beneath the plate. The measurement of the wavefront aberration of the optical system to be measured includes sensing, by the two-dimensional light-receiving element, interference between two wavefronts formed by the measurement mark, and processing the sensed interference fringe image data.
For the conventional wavefront aberration measurement, and in particular, that of the LDI scheme, it is difficult to precisely determine the processing center positions, in both the X and Y directions, to process the interference fringe image data.